The cooperation of antigen-specific cell populations in the induction of antibody formation has been well-documented in the mouse and assumed to be generally valid. The validity of this concept and the basis for T cells specificity needs to be tested by investigation of other species. The nature and function of rabbit lymphoid cells in the immne response will be examined by methods which have been used in the mouse including the primary in vitro induction of antibody formation by erythrocytes and/or hapten immunogens and the identification of lymphoid cell populations that perform different functions during an immune response. Simultaneously, antisera specific for functional classes of rabbit lymphoid cells (T cells, B cells and macrophages) will be prepared. Cellular cooperation will be assessed by recombination experiments using aliquots of cells from an individual rabbit which have been treated with the cell-specific antisera and complement. Differential irradiation will also be employed in attempts to produce T and/or B cell depleted rabbits. In vitro culture techniques and specific effects of antisera to allotypic markers and anti-immunoglobulin class sera will be used to compare the contributions of different cooperating cell populations to the Ig class of antibody produced. T and B cell populations will be examined for the presence and characteristics (class, allotype, etc.) of surface Ig and the response to different mitogens, including lectins and anti-Ig's. Since immunoglobulins are more readily identifiable on rabbit lymphoid cells than on mouse lymphoid cells, the results obtained from these experiments should clarify the role of immunoglobulin surface receptors in recognition of antigens by different lymphoid cells.